The mechanisms for explaining the reinforcing effect of a geosynthetic that includes multi-axial geogrids on a granular material such as soil or stone, for example, when a reinforced structure is used to resist the rutting effects of vehicle traffic, are still being developed. Studies, such as reported in the article entitled Aggregate Base Residual Stresses Affecting Grid Reinforced Flexible Pavement Response, Kwon et al., appearing in the International Journal of Pavement Engineering, i First article 2007, 1-11, have shown that it is not possible to describe the reinforcing effect based on the individual properties of the geogrid and granular materials alone. It is therefore necessary to introduce the concept of a composite matrix or rigid confinement layer consisting of the geogrid and the granular material that is confined and restrained by the geogrid. When a granular civil engineering structure is reinforced by a geosynthetic material of proper design having high junction strength and structural integrity, such as an integral geogrid, the resulting composite matrix is capable of withstanding increased loads and/or decreased deformations compared to an unreinforced structure.
Some of the factors that can explain the reinforcing effect of this composite matrix include (1) increase in the sustainable load due to reduced sliding and rolling of granular material due to its confinement by the geogrid; (2) increase in the shear resistance of the granular material due to frictional forces between the granular materials and the geogrid; (3) increase in the resistance to lateral bulging of the mass of granular material by the restraining effects of a stiff geogrid with strong junctions; and (4) increase in the resistance to deformation through strong mechanical bonds that form between the granular material and the rigid geogrid. The mechanical bonding, or interlocking, effect is believed to be dependent on the relative sizes of the geogrid aperture compared to the granular material.
Aperture stability modulus (ASM), a relative measure of in-plane geogrid rigidity, is one property that has been identified as useful for quantifying the reinforcing effect related to the rutting resistance of wheel loads from vehicular traffic. Actual field tests to compare the traffic performance of flexible pavements using multi-axial geogrids as reinforcement for the base courses were conducted with geogrid materials made by several methods, i.e. integral geogrids formed by punching and stretching flat sheets, integral geogrids formed from extruded and stretched grids, and geogrids formed from stitch-bonded woven fabrics. See (Webster, Steve L.; Multi-axial geogrid Reinforced Base Courses for Flexible Pavements for Light Aircraft: Test Section Construction, Behavior under Traffic, Laboratory Tests, and Design Criteria; Report DOT/FAA/RD-92; December 1992. The results demonstrated that the relative resistance of asphalt pavement test sections to rutting by a heavy wheel load correlated well with a proposed test for aperture stability modulus.
The aperture stability modulus test is carried out by clamping a sample of a multi-axial geogrid in a fixture to stabilize it and then attaching a small clamp to the ribs in an area around a junction. The junction clamp is attached to a system of pulleys and weights such that a known torque can be applied to the junction. The amount of torque required to rotate the area around the junction by a designated number of degrees is defined as the aperture stability modulus.
If carried out only on one single junction, the aperture stability modulus test can easily distinguish the relatively rigid junctions of an integral geogrid having integrally-formed junctions from a geogrid having junctions that are much less rigid when only bonded together. The aperture stability modulus also provides a relative indication of the stiffness of a geogrid. For example, a geogrid constructed by stitch bonding polymeric filaments will be quite flexible in comparison to an integral geogrid formed by punching and subsequently orienting polymeric sheet.
Because a correlation had been established relating aperture stability modulus to geogrid performance, recent design work for new geogrid structures has sometimes focused on maximizing the aperture stability modulus. In particular, U.S. Pat. No. 7,001,112 (hereinafter the '112 patent), owned by the assignee of the present invention, teaches that the aperture stability modulus is increased by approximately 65% over a conventional biaxial geogrid, having similar weight in grams per square meter, by employing six-rib structures with triangular apertures. The subject matter of the '112 patent is expressly incorporated into this specification by reference as if the '112 patent were set forth herein in its entirety.
One means of increasing aperture stability modulus, after ensuring that the grid structure possesses rigid connecting junctions or nodes, is to increase the bending stiffness of the ribs in the plane of the geogrid. The higher the resistance of the ribs to in-plane shear or bending moment, the more the ribs will contribute to the apparent “stiffness” of the aperture as measured by the techniques employed to determine aperture stability modulus. Optimum in-plane shear and bending resistance for a given mass of rib material can be obtained by choosing a low aspect ratio rib shape, where aspect ratio (AR) is defined as the most representative value of the ratio of the thickness or height of the rib cross-section to the width of the rib cross-section. To maximize the shear and bending resistance, the low aspect ratio rib, typically with AR less than one, has therefore been the preferred rib shape to maximize aperture stability modulus. The teaching of the '112 patent in fact employs a rib with aspect ratio as low as 0.38 in order to achieve high aperture stability modulus.
Low aspect ratio multi-axial geogrids have been specified in U.S. Pat. Nos. 5,156,495 and 5,419,659. In U.S. Pat. No. 5,156,495, civil engineering structures are disclosed where biaxially-oriented mesh structures have an AR of the oriented strands substantially less than unity. Finally, in U.S. Pat. No. 5,419,659, methods for constructing civil engineering structures are disclosed using biaxial geogrids where the AR of the thickness to the width of the orientated strands is substantially less than unity.
Multi-axial geogrids can be manufactured by several different methods, some of which have been used for over 25 years. Generally, such geogrids consist of ribs or strands made of oriented plastics materials. Multi-axial integral geogrids are manufactured by extruding an integrally cast sheet of polymer material which is subjected to a defined pattern of holes or depressions followed by the controlled biaxial orientation of the polymer material to cause the holes or depressions to form into apertures or mesh openings. The manufacture of such multi-axial geogrids and other integral polymer grid structures can be accomplished by well known techniques. As described in detail in U.S. Pat. Nos. 4,374,798, 4,590,029, 4,743,486, 4,756,946 and 7,001,112, a starting polymer sheet material is first extruded and then punched to form the requisite defined pattern of holes or depressions. In U.S. Pat. Nos. 3,252,181, 3,317,951, 3,496,965, 4,470,942, 4,808,358 and 5,053,264, the starting material with the requisite pattern of holes or depressions is formed in conjunction with a polymer extrusion. Other known methods for manufacturing multi-axial geogrids include, for instance, stitch bonding fabrics made of filaments, such as polyester, and applying a flexible coating, such as a PVC coating, or by weaving or by knitting, or even spot welding oriented plastic strands together. It is intended that the present invention be applicable to all types of multi-axial geogrids regardless of the method of forming the geogrid. However, integral geogrids are preferred.
Multi-axial geogrids currently in use for civil engineering structures have aspect ratios less than unity. For example, the aspect ratios of integral geogrids (with integral junctions) that are formed by punching and orienting plastics sheet are less than 1, typically in the range 0.2 to 0.9. Stitch-bonded fabric geogrids typically consist of multiple yarn bundles that are bonded by stitching and/or a coating process; the multiple yarn bundles are laid side by side to produce a composite strand of two or more adjacent yarns. Aspect ratios of these fabric geogrids typically range from about 0.1 to 0.6. Aspect ratios of grids produced by orienting extruded net structures are typically around 0.25 to 0.9. By comparison, geogrids formed by spot-welding or otherwise bonding oriented plastics strips often have very low aspect ratios, for example, less than or equal to approximately 0.2.
A high aperture stability modulus alone, however, does not necessarily constitute a multi-axial geogrid that will perform optimally when incorporated as a reinforcement or retention means for civil engineering applications. Geogrids formed by spot-welding or otherwise bonding low aspect-ratio oriented plastic strands together, for example, possess very high values of aperture stability modulus but, when incorporated into a civil engineering structure, have been demonstrated to have limited ability to resist the rutting effects of vehicle traffic when compared to integral geogrids formed with integral junctions.
Also, one can simply increase the thickness of the grid structure to increase the aperture stability modulus, but this approach adds weight and increases product cost. The preferred geogrid is one that achieves the highest amount of reinforcing effect with the lowest geogrid weight and cost.
Multi-axial geogrids have generally been formed such that the meshes or apertures have a square or rectangular shape and consist of series of parallel ribs or strands that intersect at right angles to form junctions. The ribs or strands are arranged in both a longitudinal direction, i.e. in the machine direction of the finished product as well as transversely, i.e. at right angles to the longitudinal strands. The strands usually consist of oriented polymer material in order to achieve high tensile strength with relatively low weight. Such multi-axial geogrids provide a reinforcing effect by distributing applied stresses to the longitudinal and transverse ribs.